He said: “It is in our interests for this jihadist extremist group to be stopped in its tracks. I understand entirely why people say ‘it is nothing to do with us and I don’t want to hear about it’.

But he said the jihadis “are not simply fighting Iraqis and they are also willing to fight us and they will if we don’t stop them”.

“It is vitally important that we realise what is at stake here and act. We are going to have to engage with it or the consequences will come back on us as we see in Syria today.

“The best policy for us to realise that whatever form of intervention we choose is going to be difficult but it’s better than the alternative. You do not need to engage as we did in Afghanistan or Iraq, but we do have interests in this.”

Pointing out that as many people had been killed in Syria as in the whole of Iraq since 2003, Blair said he would have supported military intervention in Syria some time ago, and suggested that there may have to be accommodation with President Assad.

His remarks drew criticism from Labour’s former international development secretary Clare Short, who accused Blair of behaving like an American neocon, adding he had been consistently “wrong, wrong, wrong about Iraq”.

She said western interventions created more tension, anger and bitterness in the Middle East, adding the invasion of Iraq “was done in such a deceitful way and with a lack of preparation for what was going to happen afterwards”.

Short said the extremists views coming out of the Middle East came from the Sunni community that was being financed by Saudi Arabia, a friend of the west and Tony Blair.

She said: “More bombing will not solve it, it will just exacerbate it”.

Alistair Burt, the former Conservative Middle East minister responsible for working with Syrian democratic forces, said non-intervention in Syria had been a disaster, just as intervention in Iraq had been a disaster.

“There is a great danger to get back to one root cause and blaming what happened in the past”, he said. “We need to find the states in the area that are going to tackle this problem because these states will ultimately threaten them and others”.

In a passionate essay published on his website, Tony Blair said it was a “bizarre” reading of the situation to argue that the US-British invasion of Iraq had allowed the growth of Sunni jihadist groups such as the Islamic State in Iraq and the Levant (Isis), whose fighters have swept through towns and cities north and west of Baghdad over the past week.

“We have to liberate ourselves from the notion that ‘we’ have caused this. We haven’t. We can argue as to whether our policies at points have helped or not: and whether action or inaction is the best policy. But the fundamental cause of the crisis lies within the region not outside it.

“We have to put aside the differences of the past and act now to save the future,” says Blair, adding that force may be necessary. “Where the extremists are fighting, they have to be countered hard, with force.”

His intervention came as the Pentagon said that the US defence secretary, Chuck Hagel, had dispatched the aircraft carrier USS George HW Bush and two guided missile ships into the Gulf as a precautionary measure.

Rear Admiral John Kirby, the Pentagon press secretary, said the Bush would be accompanied by the guided missile cruiser USS Philippine Sea and the guided missile destroyer USS Truxton. The ships were expected to arrive in the Gulf on Saturday night. Kirby described the deployment as increasing Obama’s martial flexibility “should military options be required to protect American lives, citizens and interests in Iraq”, rather than signalling an imminent strike.

In London, government officials confirmed that British military personnel could be deployed in Iraq to help tackle the growing threat to the stability of the region from Isis.

Although the Foreign Office ruled out full-scale military intervention, sources confirmed they had had discussions about sending military and police as part of a “counter-terrorism” package.

In a defence of his actions in Iraq, Blair attacked as “extraordinary” any notion the country would be stable if Saddam Hussein had stayed in power.

“The civil war in Syria with its attendant disintegration is having its predictable and malign effect. Iraq is now in mortal danger. The whole of the Middle East is under threat.”

He said it was inevitable that events across Iraq had raised the arguments over the 2003 war. While admitting that no weapons of mass destruction had been found in Iraq, he said: “What we now know from Syria is that Assad, without any detection from the west, was manufacturing chemical weapons. We only discovered this when he used them. We also know, from the final weapons inspectors’ reports, that though it is true that Saddam got rid of the physical weapons, he retained the expertise and capability to manufacture them.

“Is it likely, knowing what we now know about Assad, that Saddam, who had used chemical weapons both against the Iranians in the 1980s war – that resulted in over a million casualties – and against his own people, would have refrained from returning to his old ways? Surely it is at least as likely that he would have gone back to them?”

Blair said a likely scenario was that during the Arab spring Iraq would have been engulfed in civil war which would have blown sectarian conflict across the region. “So it is a bizarre reading of the cauldron that is the Middle East today, to claim that but for the removal of Saddam, we would not have a crisis.”

He added that until three years ago al-Qaida had been a “spent force” in Iraq and that the country had had a chance to rebuild itself. “It did not pose a threat to its neighbours. Indeed, since the removal of Saddam, and despite the bloodshed, Iraq had contained its own instability mostly within its own borders.

“Though the challenge of terrorism was and is very real, the sectarianism of the Maliki government snuffed out what was a genuine opportunity to build a cohesive Iraq. This, combined with the failure to use the oil money to rebuild the country, and the inadequacy of the Iraqi forces, have led to the alienation of the Sunni community and the inability of the Iraqi army to repulse the attack on Mosul and the earlier loss of Falluja. And there will be debate about whether the withdrawal of US forces happened too soon.”

He said that the rise of Isis was partly a consequence of the Syrian war. “To argue otherwise is willful. The operation in Mosul was planned and organised from Raqqa, across the Syria border. The fighters were trained and battle-hardened in the Syrian war.

“At its simplest, the jihadist groups are never going to leave us alone. 9/11 happened for a reason. That reason and the ideology behind it have not disappeared.”

He added: “This is, in part, our struggle, whether we like it or not.”